The present invention relates to the removal of bismuth from copper electrolyte.
Virtually all commercial copper is produced electrolytically during its production from ore or scrap. The two basic procedures are electrorefining from impure copper anodes or electrowinning following solvent extraction of copper from leach solutions.
Electrorefining consists of electrochemically dissolving copper from impure anodes and selectively plating the dissolved copper in pure form onto copper cathodes. The principal impurities in copper anodes are As, Bi, Fe, Ni, Pb, Sb, Se and Te and these separate from the anode during the process and must be removed to minimize contamination of the copper cathode product. In general, the electrolyte contains about 35-50 g/l copper and 150-230 g/l sulfuric acid.
The Se, Te and Pb are not electrochemically dissolved in the electrolyte during the electrorefining process and are removed in the anode residue slimes. The other impurities, however, are less noble than copper and tend to dissolve electrochemically from the anode along with and even preferentially to the copper of the anode. They tend, therefore, to build up in the electrolyte during electrorefining and must be removed from the electrolyte or they will eventually contaminate the cathode copper, mainly by occlusion of impure electrolyte in the growing cathode deposits. Generally, a bleed stream is specially treated in a purification section of the refinery to remove impurities and recycled to the electrolyte stream. Impurity removal from electrolyte is also a problem in electrowinning processes.
The removal of the bismuth impurity is especially important in the commercial production of copper and many methods have been developed to solve this problem.
In U.S. Pat. No. 4,157,946, which patent is hereby incorporated by reference, a method is disclosed for the selective removal of antimony and bismuth from copper refining electrolyte. The method employs adding to the electrolyte solution a salt of barium, strontium or lead to precipitate the
metal of the salt as a poorly soluble sulfate which co-precipitates the bismuth and antimony. BaCO.sub.3, Ba(OH).sub.2 and PbCO.sub.3 are exemplified in the patent.
In U.S. Pat. No. 4,179,495 the levels of impurity As, Sb and Bi are reduced in sulfuric acid solutions by adding Sb.sub.2 O.sub.3, Bi.sub.2 O.sub.3 and mixtures thereof to the solution to precipitate the impurities.
While the prior art has provided methods to remove bismuth from copper electrolyte the need still exists for a more efficient, improved method of removal.